<form [formGroup]="fromc">
  <input type="text" placeholder="child-1" formControlName="child1" (change)="makeChild2()">
  <input type="text" placeholder="child-2" formControlName="child2">
  <p style="font-size:14px;">{{makeChild}}</p>
  <div style="border:1px solid red;padding:5px;margin-top: 10px">
    子表单组件的验证：fromc.controls['child1']的value是否以 li 开头
    <br>当前表单的valid验证（child.valid）:{{fromc.controls['child1'].valid|json}}
    <br>child.errors:{{fromc.controls['child1'].errors|json}}
  </div>
</form>